1. Field of the Invention
The present invention relates to a measurement method and measurement apparatus for measuring the surface shape of a measurement target surface including an aspherical surface.
2. Description of the Related Art
As a technique of measuring the shape (surface shape) of a measurement target surface such as an aspherical lens in a non-contact manner, there is known a method using an interferometer (for example, Fizeau interferometer) with a null lens. In this method, light having a wavefront (null wavefront) corresponding to the design shape of a measurement target surface is formed via the null lens. Light (measurement light) reflected by the measurement target surface and reference light are made to interfere with each other. The difference between the wavefront of the measurement light and that of the reference light is measured, obtaining the shape of the measurement target surface. To obtain the shape of the measurement target surface at high accuracy, a measurement system error (error arising from the interferometer) needs to be calibrated at high accuracy. For example, Japanese Patent Laid-Open Nos. 10-221029, 2000-97663, and 10-281736 propose techniques regarding calibration of the null wavefront, calibration of the distortion, calibration of the magnification, and the like.
As a method of measuring the shape of a measurement target surface without using the null lens, there is also known a method using a Shack-Hartmann sensor with a large dynamic range of a measurement wavefront for a detection unit (see Johannes Pfund et. al., “Non-null testing of aspherical surfaces by using a Shack-Hartmann sensor”, “OSA”, (US), 2000, OTuC5, pp. 112-114). This method irradiates a measurement target surface with light of a spherical wave via a projection optical system.
However, in the method using a null lens, the measurement accuracy depends on the manufacturing accuracy of the null lens. Implementation of high measurement accuracy requires a long time and high cost in the manufacture of a null lens. Further, this method has other problems such that a different null lens needs to be prepared for each shape of a measurement target surface and calibration of the measurement system error becomes complicated.
In the method using no null lens, a measurement system error such as an alignment error or aberration exists in the projection optical system. Even if the detection unit can detect light (measurement light) reflected by a measurement target surface, it is difficult to separate the shape error of the measurement target surface from the measurement system error. When the measurement target surface is aspherical, light cannot perpendicularly irradiate the measurement target surface, and the incident ray angle differs from the reflected ray angle. The measurement light does not become almost parallel rays in the detection unit (because the position magnification and angle magnification of the measurement light are not constant). The measurement light is detected to have a wavefront greatly deviated from a plane wavefront, decreasing the measurement accuracy.